ARROW-4585: [C++] Add protoc dependency to flight_testing

[xhochy]

This leads to the necessary change in build.ninja

--- build.ninja.orig	2019-02-15 16:07:48.000000000 +0100
+++ build.ninja	2019-02-15 16:10:25.000000000 +0100
@@ -4863,7 +4863,7 @@
 #############################################
 # Order-only phony target for arrow_flight_testing_objlib

-build cmake_object_order_depends_target_arrow_flight_testing_objlib: phony || src/arrow/flight/CMakeFiles/arrow_flight_testing_objlib.dir
+build cmake_object_order_depends_target_arrow_flight_testing_objlib: phony || src/arrow/flight/flight_grpc_gen

[wesm]

+1. It's odd that the arrow_flight_shared dependency wasn't enough

[xhochy]

The arrow_flight_shared dependency is a link dependency but the protobuf sources are compile-dependencies. CMake auto-magically seems to know that.

[wesm]

I see, that makes more sense then. Will keep that in mind for the future =)